Chemical Equations
In a chemical equation, what are reactants and products? Where are they usually found?
Reactants are the starting substances in a chemical reaction. They are usually found on the left side of a chemical equation.
Products are the new substances formed during a chemical reaction. They are usually found on the right side of a chemical equation.
What does the Law of Conservation of Matter state?
The Law of Conservation of Matter states that matter cannot be created or destroyed during a chemical reaction.
What is the difference between an endothermic and exothermic reaction?
Endothermic reaction: Absorbs energy from its surroundings, often causing the surroundings to get colder.
Exothermic reaction: Releases energy to its surroundings, often causing the surroundings to get warmer.
What is conduction? Give an example.
Conduction is the transfer of thermal energy through direct contact between particles or objects.
Example: A metal spoon becomes hot when placed in a pot of hot soup.
Does this chemical equation accurately represent a reaction? Why or why not (2 justifications)?
2HCl + 2Na → 2NaCl + H2
Yes, this is an accurate representation of a chemical reaction.
There are 2 hydrogen atoms, 2 chlorine atoms, and 2 sodium atoms on both sides of the equation. It follows the Law of Conservation of Matter because the same number of each type of atom exists before and after the reaction.
A student claims that a rusting iron nail gains matter because it becomes heavier. Do you agree or disagree? Explain why the nail feels heavier.
Disagree; the iron reacts with oxygen in the air to form rust. The nail feels heavier because iron oxide (rust) from the surroundings adds additional mass to the nail.
Burning a candle releases light and warms the air nearby. Is the combustion reaction endothermic or exothermic?
Burning a candle is an exothermic reaction because it releases energy as light and heat, warming the surrounding air.
What is convection? Give an example.
Convection is the transfer of thermal energy through the movement of liquids or gases.
Example: Warm air rises and cooler air sinks in a room, creating convection currents.
A student writes this equation for a reaction:
H2 + O2 → H2O
What is wrong with the equation? Explain changes that would correct the equation.
The equation is not balanced because there are 2 oxygen atoms on the left but only 1 oxygen atom on the right.
If this chemical reaction obeys the law of conservation of mass, what do you expect the mass of the products to be?
Fe + S → FeS
Reactant Mass: 12(5) grams
60 grams
During a chemical reaction, the temperature of the surrounding solution drops from 24°C to 17°C. What does this suggest about the energy transfer?
The reaction absorbed heat energy from its surroundings. The reaction is endothermic.
What is radiation? Give an example.
Radiation is the transfer of energy through electromagnetic waves without requiring direct contact with a material.
Example: Heat from the sun travels through space to Earth by radiation.
N2 + 3H2 → 2NH3
How many molecules of each substance are represented? What does this demonstrate about the atoms in a chemical reaction?
N2: 1 nitrogen molecule
H2: 3 hydrogen molecules
NH3: 2 ammonia molecules
This demonstrates that atoms are rearranged, not created or destroyed, during a chemical reaction. There are 2 nitrogen atoms and 6 hydrogen atoms on each side of the equation.
A chemical reaction begins with 18 grams of reactants. One product has a mass of 7 grams, and the other product's mass is unknown. What is the mass of the second product? Explain your reasoning.
11 grams
Mass is conserved in a chemical reaction, meaning the total mass in the reactants must equal the total mass of the products.
Two reactions occur in separate containers. Reaction A releases energy to its surroundings. Reaction B absorbs energy from its surroundings. Classify each reaction as endothermic or exothermic. What happens to the temperature of the surroundings in each reaction?
Reaction A: Exothermic. It releases energy to the surroundings, so the surroundings' temperature generally increases.
Reaction B: Endothermic. It absorbs energy from the surroundings, so the surroundings' temperature generally decreases.
A metal spoon and a wooden spoon have been sitting in the same room overnight. They are both at room temperature, but the metal spoon feels colder. Why?
Even though both spoons are at room temperature, the metal spoon feels colder because metal conducts heat much faster than wood.
A student dissolves sugar in a glass of water. After stirring, the sugar can no longer be seen. Why is the sugar no longer visible? Explain your answer in terms of atoms.
The sugar is no longer visible because it dissolves and spreads throughout the water. Its particles separate from one another and mix among the water particles.
A student dissolves sugar in a glass of water. After stirring, the sugar can no longer be seen. The student claims that there is less matter because the sugar has disappeared. Do you agree or disagree? Explain what happened to the sugar and how this demonstrates the law of conservation of matter.
Disagree; the sugar has dissolved and spread throughout the water, but its particles are still present. They are mixed with the water particles rather than disappearing. This demonstrates the Law of Conservation of Matter because the total amount of matter remains the same when the sugar dissolves.
A student mixes two chemicals in a beaker. After mixing, the temperature of the mixture drops from 25°C to 16°C. The student concludes that the reaction released heat because the chemicals became colder. Do you agree or disagree? Explain what happened to the energy and identify the type of reaction.
The reaction did release heat into the surroundings. The temperature of the mixture dropped from 25°C to 16°C because the reaction released energy into its surroundings. This is an exothermic reaction.
What is heat? How is heat different from temperature?
Heat is the transfer of thermal energy from a warmer object to a cooler object.
Temperature measures how hot or cold something is based on the average kinetic energy of its particles.